Adult T-cell leukemia/lymphoma (ATLL) like most T-cell neoplasms is in dire need of novel therapeutic advances. Typical of T-cell lymphomas, ATLL presents with multi-organ infiltration and extreme treatment resistance. Unfortunately, most ATLL patients die within one year of diagnosis and current drug regimens do not extend this dismal prognosis. ATLL is initiated by infection from a complex retrovirus, Human T- Lymphotropic Virus type I (HTLV-1). After initial infection, viral proteins expressed by HTLV-1, notably tax, transcriptionally activate expression of IL2 and IL2R genes resulting in lymphoproliferation by autocrine signaling. ATLL manifests after long latency but the tax gene is usually deleted or silenced by methylation. Thus, in late stage disease, the viral stimulus of IL-2 signaling is gone but leukemic cells remain hyper- responsive to cytokines that utilize the IL2R common gamma chain (IL2RG), suggesting a role for host factors in tumor progression. In support of this idea, we recently discovered gain of function mutations in JAK3 in 11% of ATLL patients and also noted frequent overexpression and kinase activation. JAK3 is a non-receptor tyrosine kinase that binds with IL2RG to transduce signals from multiple cytokine receptors (IL2R, IL4R, IL7R, IL9R, IL15R, and IL21R) upon ligand binding. Importantly, cell lines dependent upon mutant JAK3 are sensitive to tofacitinib, a specific JAK3 inhibitor currently being tested in patients with autoimmune diseases such as rheumatoid arthritis and psoriasis. Tofacitinib has been reported to have efficacy in phase III trials of rheumatoid arthritis and is the first such compound nearing approval. In Aim 1, we will investigate the in vivo role of mutant JAK3 expression using bone marrow transduction and transplantation experiments. We will test the cooperativity of mutant JAK3 with Cdkn2a-/- and the viral tax oncogene. In Aim 2, we will treat JAK3- induced lymphomas with tofacitinib, a specific JAK3 inhibitor. In Aim 3, we will explore how negative regulators of the JAK3 pathway may be involved in ATLL pathogenesis. Our aims are well-timed and can have a significant impact on a disease that is universally fatal upon diagnosis and advance our understanding of JAK3's role in T-cell neoplasia.